The invention is related to the field of wireless power transfer, and in particular to a dual-mode wireless power receiver.
Wireless power (WP) transfer systems use the mutual inductance between two magnetic coils to transfer power through magnetic induction. These systems are commonly classified as either “inductive” or “resonant”. In a purely inductive wireless power transfer system, the source coil, which functions as the primary winding of a transformer, is driven by a voltage or current source. The receive coil, which functions as the secondary winding, is connected to a bridge rectifier, either directly or through an ac-coupling capacitor. The voltages and currents in the two windings can be determined by the relations commonly used to describe transformers.
In a resonant wireless power transfer system, the source and receiver coils are connected to capacitors to form electrical resonators. From a circuit-design standpoint, the function of the capacitors is to cancel some of the reactive impedance of the inductors, allowing more power to be transferred at a given voltage. The impedance of the inductors and capacitors varies in opposite directions with operating frequency, so the cancellation is only effective over a small range of frequencies. In other words, resonant wireless power systems utilize circuits tuned to a specific frequency at which power is to be transferred. They typically do not allow power transfer at other frequencies.
In recent years, two wireless power standards have started to emerge. The Wireless Power Consortium has released the Qi standard, which is commonly classified as an inductive charging standard. Although a resonant capacitor is used, the quality factor Q used in the Qi standard is in the low single-digits, implying that resonance is not being substantially leveraged. Devices complying with the Qi standard transmit power in the frequency range of 110-205 kHz. Qi devices typically require relatively close alignment of the source and receiver coils.
More recently, several organizations have started to introduce wireless power systems that make use of high quality factor resonant circuits to increase the usable range over which charging can occur. The frequencies used are typically much higher than those used in inductive chargers, largely due to the fact that quality factor of an inductor is proportional to frequency. One proposal for the emerging standard for resonant wireless power recommends an operating frequency in the ISM band at 6.78 MHz.